Complexes of D-, L-, DL-, and meso-tartaric acid with hydrogen and oxovanadium(IV) cations

Abstract

The formation constants of the complexes of D-, L-, DL-, and meso-tartaric acid (H₂L) with the hydrogen ion and the oxovanadium(IV) cation, [VO]²⁺, have been measured potentiometrically at 25.0 °C and I= 0.10 mol dm^–3(K[NO₃]). The enthalpy changes on formation of the protonated tartaric acids have been measured calorimetrically under identical conditions. The existence of binuclear complexes of both optically active and, to a lesser extent, meso-tartaric acids with [VO]²⁺ has been confirmed. With active tartaric acids the major species at pH >6 is [(VO)₂{D(or L)-L}₂H_–4]^4–, but with meso-tartaric acid this species is of minor importance as a result of conformational differences. The binuclear complex [(VO)₂(D-L)(L-L)H_–4]^4– is significantly more stable than [(VO)₂-(D-L)₂H_–4]^4–. This may be explained as resulting from conformational differences.